The invention is directed to maintaining the flow rate of one or more fluids proportional to the flow rate of another fluid. For example, the most common method used in the industrial heating industry for controlling air/fuel ratio of modulating-input burners is the balanced-pressure fuel regulator, supplied with a constant, fixed, upstream pressure. The balanced-pressure regulator is cross-loaded from the combustion air line, so that the outlet static pressure of the regulator is equal to the static pressure of the combustion air line supplying the burner or burners. This system works very well, provided that the minimum input to the burner or burners supplied by the regulator does not exceed about one-tenth of the maximum input to the burner or burners. The ratio of maximum input to minimum input is called turndown.
However, some burners have the capability of turndowns of more than 10 to 1. The use of the prior art fuel control system described above with burners having turndowns of more than 10 to 1 presents a difficult control problem because, with a constant upstream fuel pressure, the opening required of the balanced-pressure regulator becomes so small at the very low flows that the regulator becomes unstable.
Accordingly, it is an object of the present invention to provide a novel and improved method and apparatus for maintaining the static pressure level and flow rate of one or more (secondary) gases or liquids (fluids) proportional to the static pressure level and flow rate of another (primary) gas or liquid (fluid). It is understood that as used in the present specification and claims the term "proportional" includes "equal".